Solid lubrication by carbon-based coatings under high mechanical loads

Graphite is a well-known solid lubricant that has been studied for decades. At low loads, graphite’s lubricity depends on humidity. Classical models like e.g. the adsorption model explains this by molecular water films on graphite leading to defect passivation and easy sliding of counter bodies. To explore the humidity dependence for high loads, a commercial graphite solid lubricant was studied using microtribometry [1]. Even at very high contact pressures, a high and low friction regime is observed - depending on humidity. TEM and EELS reveal transformation of the polycrystalline graphite lubricant into turbostratic carbon. Quantum MD simulations relate high friction and wear to cold welding and shear-induced formation of turbostratic carbon. In the second part the findings are extended to CNT-based [2] and composite coatings.
[1] C. Morstein, A. Klemenz, M. Dienwiebel, M. Moseler, Nature Comm. 13, 2022
[2] T. MacLucas, et al, ACS Applied Nano Materials 6, 1755-1769 (2023)